Tuchanka

The krogan homeworld boasts extreme temperatures, virulent diseases, and vicious, predatory fauna. Around 1900 BCE, the krogan discovered atomic power and promptly instigated many intraplanetary wars, sending Tuchanka into a nuclear winter. With most of their industrial base destroyed, the krogan entered a new dark age and warring tribal bands dominated. Populations remained low for the next 2,000 years.

First contact with the salarians made resurgence possible. Krogan brought to less hostile planets bred exponentially and returned to reconquer their home. They built vast underground shelters to shield themselves from surface radiation, which proved prescient during the Krogan Rebellions when many of them isolated themselves in a vain attempt to avoid the genophage. Convinced they could outbreed the genophage, they transmitted it into more than 90 percent of the sealed bunkers. Today, Tuchanka's population is sharply limited and while individual krogan are long-lived, the genophage ensures few replacements.